Right, let's pick this up — Page 257, Lesson 248

Right, let's pick this up. We've got the DME transponder's response to interrogation, and now we're looking at what happens when too many aircraft pile in at once. First, beacon saturation. This occurs at 2700 pulse pairs per second — that's roughly 100 aircraft interrogating the same transponder simultaneously. When that happens, the transponder's receiver gain is reduced so it only responds to the strongest pulses. Think of it as the beacon being overwhelmed and shutting out the weaker signals to protect itself. Next, the station ident. The DME ground station transmits a 3-letter identifier in Morse code. Here's the key limitation: range information is not available during the ident period. So while you're hearing those letters, your DME distance readout is effectively frozen or unavailable. Now, the important part — VOR/DME frequency pairing. This is how we know which DME goes with which VOR. There are three categories. Associated — this is when the DME is co-located with the VOR, meaning within 100 feet in a TMA (Terminal Control Area) or within 2000 feet outside a TMA. In this case, the call signs are the same and the frequencies are paired. Not associated — this is when the DME serves the same location but isn't co-located. The tell-tale sign is that the third letter of the DME call sign is 'Z'. The frequencies are still paired, but the call sign tells you it's not physically with the VOR. Separated — this is when the DME is more than 6 NM apart from the VOR. Here, the call signs are different. Now, coverage. DME is line of sight — that's fundamental. It's reduced by intervening high ground and by bank angle, because when you bank, the aircraft structure can block the signal. The DOC — that's the Documented Operating Coverage — gives you the protected range. And there's an echo protection circuit that eliminates reflections, so you don't get false readings from signals bouncing off terrain or buildings. Finally, accuracy. The standard is ±0.25 NM plus ±1.25% of the range. For precision systems, it's tighter at ±0.2 NM. Now, two important errors to understand. First, slant range error. This becomes significant when the aircraft range is less than 3 times the aircraft height. That's the difference between the straight-line distance to the station and the horizontal distance over the ground. Second, ground speed error. This increases as the aircraft goes higher and closer to the station. The higher and closer you are, the more the slant range geometry distorts your computed ground speed. So, to tie it together: saturation limits how many aircraft a beacon can serve, the ident tells you which station you're locked onto, pairing tells you how the VOR and DME relate physically, and accuracy tells you how much you can trust the numbers — with slant range and ground speed errors being the two things that bite you in practice.